The present invention relates to systems for measuring distances and, more particularly, to a system wherein the distance between a transmitter and receivers at any remote location is accurately measured.
It is important in a number of application to be able to quickly and accurately determine the precise distance from a fixed base position to one or more remote locations within the line of sight of the fixed base position. One example of such an application is the surveying of land preparatory to the construction of buildings or roads where it is crucial to accurately determine distances and elevations from one or more base points to various points around the site.
A number of such systems have in the past been proposed. Typically in such systems a transmitter located at a central location transmits a single rotating beam (U.S. Pat. No. 4,268,178) or a pair of rotating beams (U.S. Pat. Nos. 3,714,657, 4,268,167, 4,911,548). Receivers at a remote location are used to sense the beam or beams which is require to allow the distance between the transmitter and receivers to be determined. What the known systems have in common is that in order to determine the distance it is necessary to have an accurate determination of the angular velocity of the transmitted beam or beams.
Typically, the angular velocity of the beam is known at the central location and is transmitted, for example by radio channel or by any other suitable means (see, for example, U.S. Pat. No. 3,714,657) to the remote location where the value of the angular velocity is used to calculate the distance between the central and remote locations.
Such systems are expensive, since they require angle to code converters, modulators, receivers, demodulator and other elements to convey the value of the angular velocity to the remote location.
In several known systems an average angular velocity of rotating laser beams is measured at the remote location rather than at the transmitter (see U.S. Pat. No. 4,268,167). This obviates the need to transmit the value from the transmitter to the remote location. However, it then becomes necessary to frequently measure the average angular velocity, for example, once per transmitter revolution. A fundamental difficulty with such a system is that the angular velocity often varies instantaneously and such variations are not picked up by the average angular velocity measurements. The result is that the accuracy of distance measurement in such systems is less than satisfactory.
There is thus a widely recognized need for, and it would be highly advantageous to have, a distance measuring system which accurately measures distances between a central location and a remote location and which is independent of the angular velocity of the beam or beams being transmitted by a transmitter at the central location.